Fluid compositions which undergo a change in apparent viscosity in the presence of a magnetic field are commonly referred to as Bingham magnetic fluids or magnetorheological materials. Magnetorheological materials normally are comprised of ferromagnetic or paramagnetic particles, typically greater than 0.1 micrometers in diameter, dispersed within a carrier fluid and in the presence of a magnetic field, the particles become polarized and are thereby organized into chains of particles within the fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the overall material and in the absence of a magnetic field, the particles return to an unorganized or free state and the apparent viscosity or flow resistance of the overall material is correspondingly reduced. These Bingham magnetic fluid compositions exhibit controllable behavior similar to that commonly observed for electrorheological materials, which are responsive to an electric field instead of a magnetic field.
Both electrorheological and magnetorheological materials are useful in providing varying damping forces within devices, such as dampers, shock absorbers and elastomeric mounts, as well as in controlling torque and or pressure levels in various clutch, brake and valve devices. Magnetorheological materials inherently offer several advantages over electrorheological materials in these applications. Magnetorheological fluids exhibit higher yield strengths than electrorheological materials and are, therefore, capable of generating greater damping forces. Furthermore, magnetorheological materials are activated by magnetic fields which are easily produced by simple, low voltage electromagnetic coils as compared to the expensive high voltage power supplies required to effectively operate electrorheological materials. A more specific description of the type of devices in which magnetorheological materials can be effectively utilized is provided in U.S. Pat. Nos. 5,284,330 and 5,277,281.
Magnetorheological or Bingham magnetec fluids are distinguishable from colloidal magnetic fluids or ferrofiuids. In colloidal magnetic fluids the particles are typically 0.005 to 0.01 micrometers in diameter. Upon the application of a magnetic field, a colloidal ferrofiuid does not exhibit particle structuring or the development of a resistance to flow. Instead, colloidal magnetic fluids-experience a body force on the entire material that is proportional to the magnetic field gradient. This force causes the entire colloidal ferrofiuid to be attracted to regions of high magnetic field strength.
Magnetorheological fluids and corresponding devices have been discussed in various patents and publications. For example, U.S. Pat. No. 2,575,360 provides a description of an electromechanically controllable torque-applying device that uses a magnetorheological material to provide a drive connection between two independently rotating components, such as those found in clutches and brakes. A fluid composition satisfactory for this application is stated to consist of 50% by volume of a soft iron dust, commonly referred to as "carbonyl iron powder", dispersed in a suitable liquid medium such as a light lubricating oil.
Another apparatus capable of controlling the slippage between moving parts through the use of magnetic or electric fields is disclosed in U.S. Pat. No. 2,661,825. The space between the moveable parts is filled with a field responsive medium. The development of a magnetic or electric field flux through this medium results in control of resulting slippage. A fluid responsive to the application of a magnetic field is described to contain carbonyl iron powder and light weight mineral oil (2-10 centipoise).
U.S. Pat. No. 2,886,151 describes force transmitting devices, such as clutches and brakes, that utilize a fluid film coupling responsive to either electric or magnetic fields. An example of a magnetic field responsive fluid is disclosed to contain reduced iron oxide powder and a lubricant grade oil having a viscosity of from 2 to 20 centipoises at 25.degree. C.
The construction of valves useful for controlling the flow of magnetorheological fluids is described in U.S. Pat. Nos. 2,670,749 and 3,010,471. The magnetic fluids applicable for utilization in the disclosed valve designs include ferromagnetic, paramagnetic and diamagnetic materials. A specific magnetic fluid composition spedfled in U.S. Pat. No. 3,010,471 consists of a suspension of carbonyl iron in a light weight hydrocarbon oil. Magnetic fluid mixtures useful in U.S. Pat. No. 2,670,749 are described to consist of a carbonyl iron powder dispersed in either a silicone oil or a chlorinated or fiuorinated suspension fluid.
Various magnetorheological material mixtures are disclosed in U.S. Pat. No. 2,667,237. The mixture is defined as a dispersion of small paramagnetic or ferromagnetic particles in either a liquid, coolant, antioxidant gas or a semi-solid grease. A preferred composition for a magnetorheological material consists of iron powder and light machine off. A specifically preferred magnetic powder is stated to be carbonyl iron powder with an average particle size of 8 micrometers. Other possible carrier components include kerosene, grease, and silicone oil.
U.S. Pat. Nos. 4,992,190 and 5,167,850 disclose rheological materials that are responsive to a magnetic field. The composition of these materials are disclosed to be either magnetizable particles and silica gel or carbon fibers dispersed in a liquid carrier vehicle. The magnetizable particles can be powdered magnetite or carbonyl iron powders with insulated reduced carbonyl iron powder, such as that manufactured by GAF Corporation, being specifically preferred. The liquid-carrier vehicle is described as having a viscosity in the range of 1 to 1000 centipoises at 100.degree. F. Specific examples of suitable vehicles include Conoco LVT oil, kerosene, light paraffin oil, mineral oil, and silicone oil. A preferred carrier vehicle is silicone oil having a viscosity in the range of about 10 to 1000 centipoise at 100.degree. F.
U.S. Pat. No. 2,751,352 and Australian Patent Specification 162,371 discloses magnetorheological fluids wherein the magnetic particles are inhibited from precipitating or settling out of the fluid system. The inhibition of particle settling is accomplished by the addition of a minute amount of an oleophobic material to the magnetic fluid. Examples of these oleophobic materials include ethyl alcohol, propyl alcohol, glycerol, ethylene glycol, propylene glycol and ethlyene diamine. The base carrier or vehicle for the magnetic particles is stated to be selected from a wide variety of materials preferably oleaginous in character. Examples of the base carrier or vehicle include mineral oils (40 to 2,000 SUS at 100.degree. F.); synthetic lubricants produced by the Fischer-Tropsch, Synthol, Synthine, Berguis, and Voltolization processes; organic synthetic lubricants; synthetic lubricants made by the polymerization of alkylene oxides at elevated temperatures in the presence of catalysts (i.e., iodine, hydrogen iodide, etc.); polymers obtained from oxygen-containing heterocyclic compounds; silicone compounds; and fiuoro and/or chloro carbon oils. While most of the base carriers or vehicles are only described as general classes of materials, specific compounds listed as carrier vehicles include light machine oil having a viscosity between 300 to 700 SUS at 100.degree. F., di(2-ethylhexyl) sebacate, di(2-ethylhexyl) adipate, ethyl ricinoleate, tricresyl phosphate, trioctyl phosphate, dibutyl trichloromethanephosphonate, trixylenyl phosphate, tributyl phosphate, triethyl phosphate, tetraphenyl silicate, tetra ethyl hexyl silicate, kerosene, and hexachlorobutadiene.
It is desirable that the continous component or carrier fluid of a magnetorheological material exhibit several basic characteristics. These characteristics include: (a) chemical compatibility with both the particle component of the fluid and device materials; (b) relatively low cost; (c) low thermal expansion; (d) high density and (e) excellent lubricity. Magnetorheological materials should also be non-hazardous to the surrounding environment and, more importantly, be capable of functioning consistently over a broad temperature range.
Most of the carrier fluid components that are traditionally used in magnetorheological materials as previously described cannot adequately meet all of these basic requirements. For instance, many of the previously described magnetorheological materials cause large variations in the force exhibited by a magnetorheological device utilizing the materials over a broad temperature range. In addition, many of these traditional magnetorheological materials provide inadequate lubricating properties between device components. Hence, many of the magnetorheological materials prepared with traditional carrier fluids limit either the useful life of a device through excessive wear or the temperataure range over which the device can be used. Conventional magnetorheological materials cannot be effectively utilized in automotive and aerospace damping devices and the like which require consistent application of precisely controlled force over widely varying temperatures.
Characterization of the performance of magnetorheological materials with respect to a change in operating temperature is vital to the successful commercialization of most magnetorheological devices, such as clutches, brakes, dampers, shock absorbers and engine mounts. All of these devices inherently experience a variation in operating temperature over their lifetime. For instance, specifications for automotive and aerospace applications typically require the device to operate at or survive exposure to temperatures ranging from about -40.degree. C. to 150.degree. C.
A need therefore exists for magnetorheological materials that are lubricating in nature and exhibit limited variation in properties over a broad temperature range.